Vibriosis: an enzootic disease of fishes

Definition:
Vibriosis is an enzootic disease of fish, recorded from all over the world. It is almost exclusively caused by halophilic vibrios of the species Vibrio anguillarum and rarely by Vibrio parahaemolyticus. It occurs among various fish species, predominantly in marine and brackish waters, and occasionally in fresh water. First described as red pest disease of eels. Now described as hemorrhagic lesions on the body surface of many marine fishes.

History:
The first records on enzootic disease among fish presumably caused by pathogenic vibrios date back in the eighteenth and nineteenth centuries. After the first description of Vibrio anguillarum from infected eels and then from cods and pikes, of the Baltic sea, the pathogen was isolated by Schaperclaus (1927) from different fish species of the bodies of water close to the Baltic sea coast.

Etiology:Vibrio anguillarum

Characteristics of Vibrio anguillarum:

• The bacterium Vibrio anguillarum is a polarly flagellated.
• Gram-negative rods.
• Vibrio species are facultative anaerobes.

Distribution:

Wide geographical distribution. Salt water fish pathogen- 12 fish families and 42 species are susceptible to Vibrio mostly V. anguillarum. Salmones are the most notable susceptible fish. V. anguillarum is found worldwide particularly along the Pacific, Atlantic, and Gulf of Mexico, coasts of North America, throughout Asia including Japan.

Host/Susceptible fishes: Channel catfish mainly at fingerling stage.

Signs and pathology:

• Skin discoloration, pale gills.
• Inactivation, cease of feeding.
• Erythema (blood blotches- redness of skin by congestion of capillaries) at the fin bases, around vent and within mouth.
• Hemorrhagic lesions on the body surface.
• Red (hemorrhagic) ulcerative necrotic skin and muscle lesions.
• Distended gut and rectum with clear viscous fluid.
• Exophthalmia.
• Swelling of spleen.

Histopathology:

• Red necrotic lesions in the abdominal muscles.
• Anemic liver and kidney.
• Pathological changes in blood connective tissues.
• Bacterial cells uniformly distributed throughout the affected tissues, greatest in blood.

Epizootiology:

• A normal marine and estuarine microbe- free living or in fish.
• Cannot survive in freshwater environment.
• Ayu infection- from carrier fish.
• Normally occur in fish in shallow waters in late summer when temperature is high and dissolved oxygen is low.
• A major component of the alimentary micro flora of healthy fishes- incubation period varies with temperature, strain virulence and degree of stress.
• Threshold temperature: 10-11⁰C for salmonids, 15-16⁰C for pleuronectids.
• Increased water temperature cause increased mortalities.
• 50% or more mortality in young fish.

Diagnosis:

• Gram negative, short, curved rods.
• Non-sporulating, non-encapsulated, motile by single polar flagellum.
• Heterogenous- five different biotypes (variant strains of a bacterial species).
• Grow in TSA or BHI by addition of 0.5-3.5% NaCl to round, raised, entire and cream-colored colonies within 24-48 hours at 25-30⁰C.
• Confirmation by biochemical tests.

Treatment:
Daily administration of antibiotics, mixed in the feed, for 10 days is usually effective to eliminate mortalities (Oxytetracycline at 100mg/kg biomass per day, flumequine at 80mg/kg biomass, oxolinic acid at 60mg/kg biomass) and potentiated sulphonamides (trimethoprim + sulfadiazine) at 70mg/kg biomass per day. However, the disease often reappears in about 20 days post a seemingly successful therapy.

Management advice:
Prevent vibriosis by a proper vaccination scheme covering all age classes of the sea bass on farm. In case of an outbreak, promptly seek veterinary advice to confirm diagnosis and suggest proper treatment measures. Reduce feeding rate even deprive feed for several days. Avoid stresses, such as handling. Remove daily and dispose off mortalities away from the farm in a proper sanitary way, approved by the local authorities. Never reject dead or moribund fish in the sea.

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